Unwanted surface mass migration has been a problem in many areas of technology, and many methods exist for preventing of alleviating the problem in particular situations. Since this applications is concerned with two particular forms of mass migration, namely, the spontaneous spreading or bleeding of a liquid, and the movement of metal ions under the influence of an electric field that is involved in the phenomenon of metal migration, typically silver migration, we will restrict our discussion to these two forms of surface mass migration.
The spontaneous spreading of a liquid on a solid surface, that is to say, the wetting of a solid surface by a liquid, has been studied extensively. See for instance, Contact Angle, Wettability, and Adhesion, Advances in Chemistry Series, 43, American Chemical Society, Washington, D.C., 1964, especially pages 1 through 49. As discussed there, spontaneous spreading occurs if the contact angle between liquid and solid goes to zero, and conversely, spontaneous spreading does not occur if this contact angle remains finite. The latter occurs when, loosely speaking, the surface energy of the solid/liquid interface is less than or equal to the difference in surface energy between the solid/vapor interface and the liquid/vapor interface. This relationship suggests that liquids will tend not to spread spontaneously on solid surfaces of suitably low surface energy. such behavior is in fact observed.
It has also been observed that coating a high energy surface with a film of low surface energy material results in a reduced tendency of liquids to spread spontaneously on this surface. Furthermore, it is known that the wettability of low-energy organic surfaces, or of high-energy surfaces coated by organic films, is determined essentially by the nature and packing of the exposed surface atoms, and is typically independent of the nature and arrangements of the underlying atoms and molecules. For instance, if a surface is covered by a thin adsorbed film comprising long-chain, unbranched, amphiphatic molecules which are able to form a close-packed array with terminal --CH.sub.3, --CF.sub.2 H, or --CF.sub.3 groups, then only liquids having relatively low surface energy will spread spontaneously on the surface. Fluorinated materials possess lower critical surface tensions of wetting than their hydrogenated counterparts, with a close-packed monolayer of --CF.sub.3 groups having the lowest critical surface tension of wetting known, about 6 dyn/cm (at 20.degree. C.).
The theoretical understanding of wetting forms the basis of some technologically important practices. For instance, poly (1H, 1H-pentadecafluorooctyl methacrylate), to be referred to herein as PDFOM, a commercially available polymer having a critical room temperature surface tension of wetting of about 11 dyn/cm, is used in industry, inter alia, as a so-called "barrier compound" for preventing the creepage and subsequent loss of lubricating oils, and for directing or "channeling" the flow of such oils, in addition to its use in blocking the spread of mobile fractions of, e.g., silicone encapsulants, and in ensuring dropwise, rather than film-type, condensation.
A typical way of using a film having a low surface energy to prevent spreading or bleeding of a relatively high-viscosity liquid such as a resin is taught by U.S. Pat. No. 4,143,456, issued Mar. 13, 1979 to K. Inoue for a "Semiconductor Device Insulation Method." That patent teaches formation of a thick film of a repellent substance on a surface in a predetermined shape around the area to which the liquid is to be confined. If the repellent material has sufficiently low surface energy then the liquid, upon encountering the repellent layer will assume a nonzero contact angle and thus will cease to spread. This approach, although being effective, has certain disadvantages. For instance, since it requires printing a repellent line around each area to which liquid is to be confined, the method is difficult to apply when a high density of components is to be adhesively affixed to a substrate, as is often the case in electronic device manufacture. And even if the component density is not high, the method still requires a relatively complex processing step, namely the printing of the patterned repellent film.
A problem that is conceptually unrelated to the wetting of solid surfaces by liquids, although, in practice, strongly affected thereby, is metal migration, in particular, silver migration. See, for instance, Saul W. Chaikin et al., Industrial and Engineering Chemistry, Volume 51(3), pp. 299-304, (1959). In metal migration, an electric field causes metal ions to move from a metallic conductor deposited on an insulator either into or along the insulator surface in the direction of increasing electrical potential, typically towards another nearby conductor that is at a lower electrical potential than the first. In electronic equipment using printed conductors, for instance, this phenomenon is highly deleterious, since it can result in short-circuiting.
Since of all commonly used conductor materials silver is most susceptible to metal ion migration, the phenomenon has been mostly studied in the form of silver migration. Various approaches have been used to prevent or reduce silver migration, such as, for instance, coating the silver-containing conductor with gold, solder, or other materials, treating the insulator surface, or alloying the silver. In particular, since it appeared likely that reducing adsorption of water on the insulating substrate surface would reduce the intensity of silver migration on normally susceptible materials, Chaikin et al. have used a silicone-product to treat a melamine-glass laminate substrate, and this treatment was found to reduce silver migration by roughly 75% (i bid, page 302). A somewhat different approach is taught by U.S. Pat. No. 3,909,680, issued Sept. 30, 1975 to E. Tsunashima, for "Printed Circuit Board with Silver Migration Prevention." The patent teaches use of an undercoating and/or an overcoating comprising insulating resin and an organic inhibitor.
Both the method of preventing silver migration by means of treating the insulator surface with a silicone product and that of using coating layers comprising an inhibitor require additional processing steps, and furthermore result in modifications of the surface that typically, inter alia, affect the ease of component replacement.
Because of the shortcomings of prior art methods for preventing unwanted spreading of liquids as well as of those for preventing metal migration on surfaces, convenient, inexpensive, and reliable methods for achieving these ends that do not substantially impair any desirable properties of the surface are of considerable interest.